The present invention relates generally to induction-type rotational position detecting devices and more particularly to an induction-type rotational position detecting device where the rotor has no windings and rotary transformer.
Among various induction-type rotational position detecting devices, those which are designed to produce two-phase (sine phase and cosine phase) outputs in response to a single-phase exciting input are commonly known as "resolvers", and those which are designed to produce three-phase outputs (phases shifted 120.degree. in relation to each other) in response to a single-phase exciting input are known as "synchro" devices. The oldest-fashioned resolvers have double-pole (sine pole and cosine pole) secondary windings provided on the stator in such a manner to cross each other at a mechanical angle of 90.degree., with a primary winding provided on the rotor. However, the conventional resolvers of this type are disadvantageous in that they require brushes for electric contact with the primary winding on the rotor. Brushless resolvers eliminating the need for such brushes are also known, where a rotary transformer is provided on the rotor in place of the brushes. Because of the provision of the rotary transformer, the size of the resolver can not be reduced easily or can be reduced only to a limited degree. Also, the provision of the rotary transformer would result in an increased number of component parts necessary for the resolver and hence increased costs.
Non-contact/variable-reluctance-type rotational position detecting devices have been known under the tradename "microsyn", in which primary and secondary windings are provided on a plurality of projecting poles of the stator, and the rotor is formed of a magnetic body having a predetermined shape (eccentric circular or oval shape, or a shape having a projection). In such non-contact/variable-reluctance-type rotational position detecting devices, the gaps between the stator's projecting poles and the rotor's magnetic body are caused to vary in response to a changing rotational position of the rotor with reluctance variations occurring in response to the changing rotational position of the rotor, so that there are generated output signals corresponding to the reluctance variations. Also, similar rotational position detecting devices based on the variable reluctance principle are disclosed, for example, in Japanese Patent Laid-open Publication Nos. SHO-55-46862, SHO-55-70406 and SHO-59-28603. In connection with such devices, a phase-based position detecting method (where detected position data corresponds to an electrical phase angle of the output signal) and a voltage-based position detecting method (detected position data corresponds to a voltage level of the output signal) have both been known as methods for detecting positions based on the output signals of the devices. In the case where the phase-based position detecting method is employed, primary windings disposed at different mechanical angles are excited by plural-phase (e.g., two- or three-phase) exciting inputs to generate a single-phase output signal which varies in electrical phase angle in response to a changing rotational position of the rotor. In the case where the voltage-based position detecting method is employed, the relationship between the primary and secondary windings is reversed from the above-mentioned phase-based method, and plural-phase outputs (or a single-phase output having a peak amplitude level corresponding to a changing rotational position of the rotor) are generated in response to a single-phase exciting input as in the above-discussed "resolvers" or "synchro devices".
However, the above-mentioned non-contact/variable-reluctance-type rotational position detecting devices were unable to achieve good linearity of detected position data and high detecting accuracy because no winding means, such as primary and secondary windings and a rotary transformer, were provided on the rotor, although they were more suitable for achieving reduced size as compared to the conventional resolvers.
According to the study by the present inventor et al., one of the reasons for poor detecting accuracy of the prior art rotational position detecting devices where no winding means such as primary and secondary windings and a rotary transformer are provided on the rotor, is probably that the degree of magnetic coupling between the primary and secondary windings does not vary ideally in proportion to a changing rotational position of the rotor. For example, in the conventionally known variable-reluctance-type rotational position detecting devices, the rotor made of a magnetic body is formed into an eccentric circular, oval or gear-like shape or the like so that reluctance in a magnetic circuit passing through a given magnetic pole of the stator is caused to vary as the gap between the end of the magnetic pole on the stator and the rotor's magnetic body changes in response to a changing rotational position of the rotor. On the basis of the reluctance variation, the degree of magnetic coupling between the primary and secondary windings at the stator's magnetic pole changes in response to the rotational position, which induces in the secondary windings output signals corresponding to the rotational position.
In such a form of induction, variation in reluctance or in the degree of magnetic coupling based on the gap change at one point of a given magnetic pole of the stator would appreciably influence the detecting accuracy of the device, and thus high-accuracy detection output with good linearity could not be constantly obtained throughout an rotation of the rotor. Various attempts to solve these problems have been proposed to date, one typical example of which is to form the rotor into a special shape such as a heart or similar shape. Even though such an attempt may significantly improve the detecting accuracy, it does not appear so useful since it would require cumbersome designing as well as delicate manufacturing accuracy. Further, the fact that the gap change at one point of a given magnetic pole of the stator influences the detecting accuracy causes another problem that considerable manufacturing and assembling accuracy would be required for each of the magnetic poles of the stator.